If you can run the mail server yourself, and have that forward to your mail ISP, then you might be able to do this from an Arduino WiFi shield. The reason it's unlikely you could send the alarm directly from the WiFi shield to your ISP is because modern mail sending protocols require a lot of processing power (often using TLS/SSL, check-before-send authentication, etc.)A Raspberry Pi should have no problem doing that, assuming you configure it right.